Currently, sputtering techniques are the primary method used for depositing thin film metallic interconnects and adhesion layers, particularly for the manufacture of VLSI devices. As technology advances with respect to the manufacture of semiconductor devices, the semiconductor wafers are becoming larger, the line dimensions smaller, the number of transistors is increasing and the circuits are becoming more complex. These advances are requiring additional metallization layers and tighter tolerances. Connecting circuits are often created in multiple steps. For each metallic layer of the connecting circuit, a sequence of uniform thin films are sputtered onto the surface of the patterned semiconductor wafer. The sputtered layers are subsequently etched to form the required circuits.
Targets used in present day sputtering systems are generally conditioned or burned-in before they can be used for product wafers. The burn-in time and other variables are dependent on a particular sputtering system to be used. The purpose of the burn-in process is to remove any residual contaminants adsorbed or absorbed on the surface of the target which may adversely affect the quality of the sputtered film and reduce particle levels to acceptable limits. The residual contaminants can occur as a result of the packaging, shipment or storage of the target. Other contaminants intended for removal during burn-in includes the residual surface oxide layer on the target and any areas of targets which may be damaged by the target's final machining process. The burn-in process should leave a clean metal surface ready for sputtering on to production wafers.
During the burn-in process, outgassing and particle generation may occur, particularly if contaminants exist on the target. This outgassing and particle generation may result in contaminants or other foreign elements and particulates being trapped in the sputtering chamber or deposited onto the shields of the apparatus. These residual contaminants may then impact the performance of the sputtering operation. The particles and other impurities are critical factors in integrated circuit manufacturing which impact the device yield directly. The particles and impurities must be kept at levels as low as possible to maximize the device yield.
The burn-in processing is a non-value step as part of the sputtering process. This non-value step impacts the entire manufacturing process and can contribute to increased cost of ownership. To illustrate, sputtering systems are very expensive (about $3 million) and downtime on this equipment is estimated at $0.5 million per hour (.about.$10,000 per wafer at 50 wafers per hour). Burn-in of a new target generally takes 1 to 6 hours that can not be used for production. Reducing burn-in time will result in significant savings and reduction of cost of ownership.
In view of the disadvantages associated with the need to burn-in a target, i.e., increased manufacturing time and possible adverse effects on the sputtering operation and the manufactured product yield, a need has developed to improve the sputtering target processing sequence to reduce the burn-in time and improve the overall manufacturing process and process yield.
In response to this need, the present invention overcomes the disadvantages noted above by providing a method which surface treats a manufactured sputtering target and encloses the surface treated portion in a protective metal enclosure (either packaging or coating) prior to burn-in. With this invention, reductions in both burn-in time and the generation of contaminants are realized. In one application of the invention burn-in time for an aluminum target was reduced from 5-6 hours to less than one hour. At this point particulate was minimized and processing fully stable.